The selective estrogen receptor modulator (SERM) tamoxifen (TAM) is the most widely used endocrine therapy for the treatment and prevention of estrogen receptor alpha (ER1) positive breast cancer. However, ~ 40% of initially TAM-sensitive tumors become endocrine/TAM-resistant. The mechanism behind such acquired TAM resistance is unknown and biomarkers of TAM-response may be useful to monitor clinical response. MicroRNAs (miRNAs) are a class of naturally-occurring, small, non-coding RNA molecules that are involved in regulating the translation and processing of mRNAs, usually by binding to the 3' untranslated region of target mRNAs and targeting the mRNA transcript to be degraded or by blocking translation. The human genome contains > 700 miRNAs. Aberrant patterns of miRNA expression have been recently implicated in human disease with miRNAs differentially expressed in concordance with other well-established markers of breast cancer stage and patient prognosis including ER1 and progesterone receptor, tumor stage, number of positive lymph nodes, and vascular invasion. Using the NIH-R21 mechanism, we obtained preliminary data identifying miRNAs regulated by estradiol (E2) in an ER1-dependent manner in MCF-7 human breast cancer cells and identified downstream target genes that were upregulated via E2 downregulation of miR-21. However, to date, no one has examined TAM affects the pattern of miRNA expression in human breast cancer and only 2 reports have identified miRNA expression patterns in TAM-resistant derivatives of MCF-7 breast cancer cells. Specific Aim 1 is to identify miRNAs that are differentially regulated by E2 and 4-hydroxyTAM (4-OHT) in antiestrogen- sensitive MCF-7 and T47D breast cancer cells. Specific Aim 2 is to identify miRNAs and their target genes in antiestrogen/ TAM- sensitive versus -resistant breast cancer cell lines and tumor xenografts. This Aim tests the hypothesis that miRNA expression is dysregulated in endocrine/TAM- resistant versus -sensitive breast cancer cells. Specific Aim 3 is to determine if the E2- regulated and 4-OHT- regulated miRNAs identified in breast cancer cell lines show aberrant expression in human breast tumors and correlate with clinical diagnostic measures and patient response to tamoxifen therapy. The overall goal of the proposed research is to determine the identity and gene targets of miRNAs that may provide novel biomarkers and new insights into the mechanisms by which breast tumors gain endocrine/TAM-resistance and become invasive and metastatic.